[processing][saga] Fix cubic spline definition, add unit test

(cherry-picked from 43d3870)

Author: nyalldawson

python/plugins/processing/algs/saga/description/CubicSplineApproximation.txt

@@ -5,9 +5,9 @@ QgsProcessingParameterField|FIELD|Attribute|None|SHAPES|-1|False|False
 Hardcoded|-TARGET_DEFINITION 0
 QgsProcessingParameterNumber|NPMIN|Minimal Number of Points|QgsProcessingParameterNumber.Integer|3|False|0|None
 QgsProcessingParameterNumber|NPMAX|Maximal Number of Points|QgsProcessingParameterNumber.Integer|20|False|11|59
-QgsProcessingParameterNumber|NPPC|Points per Square|QgsProcessingParameterNumber.Integer|5|False|1|None
-QgsProcessingParameterNumber|K|Tolerance|QgsProcessingParameterNumber.Double|140.0|False|0|None
+QgsProcessingParameterNumber|NPPC|Points per Square|QgsProcessingParameterNumber.Double|5|False|1|None
+QgsProcessingParameterNumber|K|Tolerance|QgsProcessingParameterNumber.Integer|140|False|0|None
 QgsProcessingParameterExtent|TARGET_USER_XMIN TARGET_USER_XMAX TARGET_USER_YMIN TARGET_USER_YMAX|Output extent|None|True
-QgsProcessingParameterNumber|TARGET_USER_SIZE|Cellsize|QgsProcessingParameterNumber.Double|100.0|False|None|None
+QgsProcessingParameterNumber|TARGET_USER_SIZE|Cellsize|QgsProcessingParameterNumber.Double|1.0|False|0|None
 QgsProcessingParameterEnum|TARGET_USER_FITS|Fit|[0] nodes;[1] cells
 QgsProcessingParameterRasterDestination|TARGET_OUT_GRID|Grid

python/plugins/processing/tests/testdata/saga_algorithm_tests.yaml

@@ -411,3 +411,21 @@ tests:
         name: expected/saga/snap_points_to_points_result.shp
         type: vector
 
+  - algorithm: saga:interpolatecubicspline
+    name: Cubic spline
+    params:
+      FIELD: fid
+      K: 140
+      NPMAX: 20
+      NPMIN: 3
+      NPPC: 2.0
+      SHAPES:
+        name: pointsz.gml
+        type: vector
+      TARGET_USER_FITS: 0
+      TARGET_USER_SIZE: 0.5
+    results:
+      TARGET_OUT_GRID:
+        hash: f4ac677152697773262aa1e5f69146a475139014ace77149966ec045
+        type: rasterhash
+